Estimation and Simulation of Gas Permeability as well as Stress-Strain Behaviour of some Indian Coal Seams

Abstract

Coal bed methane (CBM) is considered as unconventional gas, and plays an important role in fulfilling the future energy supply as well as repositories for CO2 storage. The recovery of methane is a promising technology to meet a part of the growing energy demand. CBM is considered as clean source with minor amounts of carbon dioxide which is 117 lb/MBtu as compared to coal which produce 228.6 lb/MBtu, though its occurrence and extraction poses many challenges. CBM production from coalbed reservoir is typically initiated using water depletion method. This method reduces the pressure in coal bed as water is pumped out from the reservoir. Reduction in pressure in coal bed reservoir leads to desorption of gas from the coal matrix and flow in to the production well. Desorption of methane produces voids in coal matrix that facilitate repositories for CO2 storage. The commercial and economic extraction of methane and sequestration of CO2 depends on the various parameters of the coal seam including proximate and ultimate analysis parameters, petrographic parameters, pore size distribution, microstructural analysis, mechanical properties, in situ gas content, adsorption/desorption characteristics and gas permeability characteristics etc. A significant change in these parameters adversely affects the mechanism of CBM production economically. The influence of gas permeability is the most important parameters for determination of gas flow from matrix to well and vice versa. Permeability is highly susceptible to the mechanical properties. A significant change in mechanical properties alters the flow mechanism i.e. gas permeability in coal seam. Therefore, it is very important to evaluate these parameters in order to accurately evaluate the rate and amount of methane recovery and injection of CO2 in deep coal seam.
In this research work exhaustive investigation was carried out to determine the parameters of coal characterization, adsorption/desorption characteristics and gas permeability characteristics of the coal at 400 to 580 m depth obtained from Jharia coal field (Moonidih Area). Proximate and ultimate analysis parameters, petrographic analysis parameters, Microstructural analysis, pore size distribution and mechanical properties etc. were evaluated for coal bed reservoir characterization. The variation of proximate analysis parameters like moisture content, ash content, volatile matter and carbon content confirms medium to low volatile bituminous coal. Reduced moisture content from 1.4 to 0.93 wt. % and higher fixed carbon from 51.08 to 71.13 wt. % with increase in depth from 400 to 580 m confirms the higher porosity and gas adsorption site in coal matrix at maximum depth of occurrence. Ash content was observed below 10 % at 580 m depth exhibits increase in hydrocarbon content (methane) with depth. The variation of ultimate analysis parameters like carbon, hydrogen, nitrogen and sulphur confirms bituminous rank of coal. The variation of carbon and hydrogen content from 66.74 to 84.88 wt. % and 3.77 to 6.23 wt. % exhibits the enrichment of vitrinite macerals and methane content in coal seam with depth. Increase in vitrinite macerals from 46.58 to 58.94 % confirms the increase in methane content of coal seam with depth. The calculated vitrinite reflectance from 1.07 to 1.37 % lies in between the threshold value 0.7 to 2 % reflects commercial prospects of CBM in study area. The pore size distribution illustrates the presence of meso and macro pores in coal. Maximum frequency of pore size from 10 to 50 nm in all coal samples confirms the thermogenic gas generation process. The SEM images indicate the presence of primary and secondary pores in coal. The distributions of pores were dispersed or aggregated, circular, elliptical, and lenticular or strip indicates strong adsorption capacity of hydrocarbons. Inter particle as well as intra particle pores were distributed throughout the coal samples significantly impacting hydrocarbon storage. The presence of micro fractures at the edges of the mineral components confirms the existence of flow channels between micropores and macropores for flow of gas. Unconfined compressive strength, elastic modulus and poisson’s ratio varied from 1.91 to 6.51 MPa, 0.42 to 0.54 GPa and 0.32 to 0.39 at 400 to 580 m depth respectively. Decreased in strength was observed with depth and is attributed to reduced microlithotype as well as carbominerites in coal at higher depth.
Maximum in situ gas content of 12.13 cc/g confirms good prospects of coal bed methane in the study area as it exceed the threshold value of 8.5 cc/g for commercial production of CBM. Maximum gas storage capacity of 20.52 cc/g for CO2 and 13.29 cc/g for CH4 was observed at 580 m depth. Increase in gas storage capacity with depth attributed reduction in moisture and ash content in the coal, resulting in more sites available for gas adsorption. The adsorption capacity of CO2 was higher than that of CH4 because of the smaller molecular size of CO2 (0.33 nm) than CH4 (0.38 nm). Higher CO2 adsorption capacity of coal confirms promising sequestration potential of coal bed reservoir. Decrease in adsorption capacity of coal with temperature exhibits the reduction in adsorption capacity of coal at higher depth of occurrence. The determination of permeability of coal samples at variable confining pressure exhibits more than 84% reduction in permeability with increase in confining pressure from 1 to 6 MPa. Reduction in permeability was fast from 0 to 3.92 MPa but it was slower from 3.92 to 6 MPa. The reduction in permeability is due to crushing of grain as well as narrowing and closer of fractures. The results of reservoir simulation over twenty five year of well life illustrate the maximum gas rate of 10358 m3/day and cumulative gas volume of 6.35 x 107 m3.
Scientific novelty of the findings consist the characterization of coal seam based on the laboratory experimentation, determination of the adsorption/desorption capacity of coal and its variation with depth and temperature, fluctuation of permeability with gas and confining pressure, mechanism of gas permeability, forecasting of rate of production and cumulative gas volume etc. for the industrial importance and commercial CBM production as well as CO2 sequestration in deep coal seam.